1. Field of the Invention
The invention relates to a circuit arrangement for detecting the capacitance or change of capacitance of a capacitive circuit element or of a component, with a control unit, with a monostable multivibrator triggered by the control unit, and with an evaluation unit, the monostable multivibrator having a first input connected to the control unit, a second input connected to the capacitive circuit element or component, and when the monostable multivibrator is triggered by the control unit at the output of the monostable multivibrator, an output signal or output signals appears with a time duration which is proportional to the capacitance of a capacitive circuit element or component, the output signal or output signals of the monostable multivibrator being converted into a signal voltage proportional to its or their duration, and in the evaluation unit, a detection value corresponding to the capacitance or change of capacitance of the capacitive circuit element or component being obtained from the signal voltage.
2. Description of Related Art
Within the framework of the invention “capacitance” means the capacitance value of a capacitive circuit element or component; “change of capacitance” consequently means a change of the capacitance value of a capacitive circuit element or component, “detection” of the capacitance or change of the capacitance within the framework of the invention means both only qualitative detection and also quantitative detection, therefore a genuine measurement. A “capacitive circuit element or component” within the framework of the invention means any circuit element and any component which has capacitive properties, often also called capacitance, then the capacitance value not being intended. A “capacitive circuit element or component” is especially a capacitor.
However, a “capacitive circuit element or component” within the framework of the invention also means the electrode of a capacitive proximity switch, in interaction with an influencing body. A “capacitive circuit element or component” within the framework of the invention also means, for example, the capacitance which is constituted by lines which capacitively interact with one another. Instead of a “capacitive circuit element or component,” a sensor capacitor is also addressed without thus being linked to a limitation to a capacitor in the narrower sense.
Multivibrators are positive feedback digital circuits (see, Tietze/Schenk Semiconductor Circuit Engineering, 12th edition, page 600). They differ from the positive feedback linear circuits (oscillators) in that their output voltage does not change continuously, but jumps back and forth only between two fixed values. The flipping process can be triggered in different ways. A monostable multivibrator which belongs to the circuit arrangement of the type to which the invention is directed has only one stable state. The second state is stable only for a certain time interval which is fixed by dimensioning. After this time interval expires, the circuit flips back into the stable state by itself. Therefore, it is also called a time switch, monoflop, univibrator or oneshot. Monostable multivibrators can be implemented quite differently (see, for example, Tietze/Schenk, op. cit, pages 603 and 604, 608 and 609 and 618 to 620). For their implementation, especially logic components can be combined with one another, for example, XOR, coincidence gate, AND, NARD, NOR, OR or/and a flip-flop.
In the known circuit arrangement underlying the invention (see, DE 102 58 417 B3), the control unit, a clock generator, produces a low-frequency clock sequence, for example, with a clock frequency of 1 kHz, and a lowpass filter is connected downstream to the output of the monostable multivibrator and the signal voltage can be tapped at the filter output.